The device makes it possible to determine, in a mixture of particles or grains of various sizes, the percentage of particles whose sizes are contained between predetermined succesive ranges. The mixture is preferably in powder form and it is then transformed into a suspension of particles in a gaseous, or, more often, liquid fluid.
The suspension must be sufficiently diluted and formed as a sufficiently thin layer to enable light to pass through it and be partially diffracted by the particles.
However, the mixture can assume other forms, e.g. a biological preparation fixed on a transparent plate.
The device in accordance with the invention uses the known method of illuminating the mixture with a parallel beam of monochromatic light, forming a diffraction pattern, examining the pattern, which has circular symmetry about a centre to determine the distribution of the light flux as a function of radius (distance from the centre) and mathematically processing the signals supplied by the examination to deduce therefrom data concerning the granulometric composition. It is assumed here that the particles are spherical. Actually, they may have other shapes which can be observed beforehand by microscope examination. However, the person skilled in the art can then experimentally establish an at least approximate equivalence between each diameter of a spherical particle and a value of a characteristic size of a particle of known non-spherical shape.
For example, uses of this method are known through: French patent application No. 71 30 802 filed on Aug. 25, 1971; the corresponding to U.S. Pat. No. 3,807,864; the first French certificate of addition no. 77 27 308 to the above French patent application; an article by J. Cornillault entitled "Particle Size Analyzer" (Applied Optics Vol. 11, No. 2, February 1972, p. 265-268) and articles by Wertheimer et al. entitled "Light scattering measurements of particle distributions" (Applied Optics, vol. 15, p. 1616, June, 1976); and "Light scattering instrumentation for particulate measurements in processes" (SPIE, vol 129., Effective utilization of Optics in quality assurance, 1977).
These documents describe known methods of measuring not the illumination of the diffraction pattern for each value of the radius, but the total value of the light flux received on several "integration areas" which are each constituted, for example, by an opening formed in a screen and followed by a light detector which supplies a detection signal proportional to the flux. Such a disposition makes it possible for each light detector to receive a light flux which is sufficient for it to be measured easily and accurately. Above all, it allows an improvement in the quality of the final results, subject to a suitable choice of the shapes and dimensions of the integration areas. This is explained as follows: consider the light flux received on a very thin circular ring which is concentric to the pattern and has an inner or outer radius d, the flux is divided by the very small difference between the inner radius and the outer radius of the ring. The result of the division is a function of the radius d. This function will here be called f(d). The same function f(d ) can be more precisely defined as the derivative of another function of d, here called F(d) which represents the flux received inside the circle of radius d.
Each integration area extends between an inner circle of radius d1 (inner radius) and an outer circle of radius d2 (outer radius). For each value of the radius d lying between d1 and d2, the integration area, as seen from the centre of the diffraction pattern, has an angular extent A (angle at the centre) which may be constant but which may alternatively vary when the radius d changes. The angular extent can therefore be considered as a function A(d). The total flux Fk received in an integration area Sk is then equal, to within a constant, to the integral of the product A(d).F(d), with respect to the variable d, between the limits d1k and d2k, representing the values of d1 and d2 for the integration area. The values of d1k and d2k and possibly the law of variation of the angular extent 1, can be chosen at will so as to obtain from each integration area a detection signal which makes it possible to obtain final results which are as useful as possible, taking into account the aims to be achieved.
These aims can be diverse, but it is often required to obtain a histogram (or a continuous granulometric curve which contains the same data as a histogram), i.e. there is defined a continuous sequence of ranges of values of the diameter a of the particles from the smallest possible diameter to the greatest, e.g. in a geometrical progression and the percentage of the number of the particles whose diameters are contained in each of these successive ranges is to be found. There result two conflicting requirements. One is fineness of measurement, i.e. the number of ranges must be as great as possible, each of the ranges being as narrow as possible. The other requirement in independence of measurement, i.e. the percentage measured for a range must be altered as little as possible by the presence of particles whose diameters are included in the other ranges. In accordance with known dispositions, unfortunately, these two requirements cannot be achieved simultaneously, since if an increased number of narrower ranges is chosen, the percentage measured for a range will be more greatly affected by the presence of particles which correspond to several neighbouring ranges. This can be explained considering the distribution of light sent to the diffraction pattern by particles of a given diameter a, the greater part of said light being distributed from the centre of the spot up to a distance d from the centre. The smaller the diameter a, the greater the distance d. Therefore, to estimate the percentage of particles in a range with a diameter of about a in a mixture, it is necessary to measure the light flux in a zone of the diffraction pattern (integration area) which is sufficiently far from the centre so as not be hindered by light diffraction by the particles whose diameters come within the higher ranges and said zone must be sufficiently near to gather a significant fraction of light diffracted by the particles sought. However, light diffracted by the particles of greater diameter and especially light diffracted by particles of smaller diameter does cause slight disturbance but does not make it impossible to estimate the percentage of the particles in the range of diameters considered, since the influence of the other particles can also be taken into account when the received signals are mathematically processed, but it results in inaccurate estimation and the greater the proportion of hindering light, i.e. light diffracted in the zone in question by these other particles, the greater the inaccuracy. It is well known in metrology that such inaccuracy is observed each time an estimation is made by subtraction of two measurements which are themselves erroneous and cannot be corrected and that this inaccuracy is increased when these two measurements are close to each other.
Referring again to measurement of granulometry by diffraction of monochromatic light, there are three possibilities for keeping accuracy within acceptable limits, i.e. for keeping the estimation of the percentage of particles in one range of diameters sufficiently independent from the percentages of the other particles. A first possibility consists of choosing sufficiently large values for both the relative width of the diameter ranges (i.e. in practice the ratio between the largest and the smallest diameters in the range) and the radial extent of the areas of integration (i.e. the ratio between the greatest and smallest radius of the area). This makes it possible simultaneously to collect a large fraction of the light diffracted by the particles in the diameter range in question and to collect only a relatively smaller fraction of the light diffracted by the other particles. However, this possibility is limited by the fact that the width of the ranges becomes excessive when using the results. In practice, it is possible to continue in this way up to a relative width equivalent to 2; then the user requires a width whose value is at least that of the square root of 2, i.e. about 1.4.
A second possibility, which can be used in equipment found in trade, consists in disregarding the finest particles which cause the greatest hindrance. In practice, this is equivalent to either studying mixtures which do not include particles with a diameter of less than about one micrometer or, if the mixture includes such particles, not measuring their concentration and, what is worse, neglecting their influence, i.e. finally accepting erroneous estimations for the concentrations of particles of diameters greater than one micrometer.
A third possibility takes into consideration the fact that the hindrance due to the finest particles is connected not directly to the diameters thereof, but to the ratio between their diameters and the wavelength of the light used. This possibility consists in using light with a shortened wavelength. In practice, helium-neon laser light is used at present, its wavelength being 0.63 micrometers. This would lead to using a clearly shorter wavelength, e.g. twice as short. This would be both complex and expensive.
The present invention aims to produce a device for determining the granulometric composition of a mixture of particles by light-diffraction to make it possible in particular to obtain simply and accurately a granulometric histogram with sufficiently narrow ranges, even when the mixture studied includes particles of diameters close to the wavelength of the light used.